A stack of wellbore blowout preventers (BOP stack) incorporated with the riser piping of a wellbore casing system maintains and controls pressure in the wellbore of an oil and gas well. Often it is necessary to perform well workover operations such as snubbing. Snubbing is the process of running or inserting pipe into the wellbore with specialized equipment called a snubbing unit when the blowout preventers of the BOP stack are controlling wellbore pressure. A snubbing unit is typically comprised of traveling and stationary slips, hydraulic jacks, a work window and a workbasket that extends above the BOP stack. The workbasket carries the workers used to perform the snubbing operations and also carries the pipe handling equipment, such as a tong arm, tongs, a gin pole, and their associated controls, and the controls necessary to operate the BOPs, the traveling and stationary slips, and the hydraulic jacks. A snubbing unit can also be used for a variety of well operations including fishing, milling, drilling, side tracking, and installing and removing bridge plugs when a drilling rig is not available.
In an onshore application, ground anchors with attached guywires and cables are used to support the workbasket, snubbing jack, and work window of a snubbing unit mounted on an associated BOP stack. At an offshore location, the workbasket, snubbing jack, and work window of a snubbing unit and the associated BOP stack are supported by guywires or cables that extend to a platform deck or to a temporary framework attached to the platform, or to the outer casing string, or conductor pipe. The temporary support framework must be prepared before installation of the snubbing unit. Engineering analysis must be performed to verify the structural integrity of the temporary framework.
Often the temporary support framework must be tailored and fabricated at the well site before the snubbing unit can be set up for operation. This tailoring and fabrication at the well site increases the time and expense associated with installation of a snubbing unit and may exposes workers to an increased risk of injury. Engineering analysis of the temporary support framework may have to be repeated. Often additional engineering analysis is not performed. After the job is completed, the temporary framework must be removed which adds additional cost to the project and to risk of worker injuries.
The conventional temporary support framework at an offshore well site is typically more economically installed ahead of the workover unit and crew using a small liftboat or other type of jackup vessel. When a larger jackup vessel was previously used at the same location, the ability to position a smaller jackup vessel is sometimes limited. The spud can holes of a jackup vessel leave a footprint on the water bottom. These footprints can cause significant structural problems when a smaller jackup vessel having different leg spacing is jacked up in the same area. To prevent the legs of the smaller jackup vessel from sliding or shifting into the spud can holes of a previous vessel, the smaller jackup vessel must be kept a safe distance away from the can hole footprint. Encroachment on the footprint of a larger jackup vessel can cause the smaller vessel to shift and lose its position on the water bottom. A shift in position can cause significant leg damage to the smaller jackup vessel and in extreme cases a leg may break and topple the vessel. This restriction on positioning may impede the installation of the temporary support platform if the liftboat or jackup vessel being used for installation of the temporary platform has limited crane capabilities. The vessel may have to be jacked down and moved over to the opposite side of the wellhead platform and repositioned for installation of the temporary support structure beams. Adverse weather also has an impact on the use of liftboat or jackup vessels, and such weather can delay the rigging of temporary offshore support platforms when the use of a liftboat or jackup vessel is required.
If welding is required for installation of a temporary support structure, adjacent producing wells will likely have to be shut in to guard against explosion and fire and to safeguard personnel and property. Once wells are shut, it is often difficult to bring a shut in well back on line and further well intervention such as nitrogen lifting, wireline swabbing, or the like may be required to place the well in service. The anticipated loss of production caused by having to shut in producing wells may be the driving factor in not repairing a non-producing well on the same platform and valuable oil and gas may be left in the ground at the end of field life.
When rigging up a temporary support structure in the conventional manner, engineers and planners try to prevent eccentric or side loads on the support structure beams. However obstructions at a well site such as platform hand rails, flow lines, ladders, and production facilities will often require the temporary support structure beams to be placed at awkward angles. When this occurs, the support structure beams may have to be upsized to handle the eccentric loading. In many cases these obstructions cannot be fully determined until the construction crews arrive at a platform. This can slow down installation of the temporary support structure or result in using support structure beams which do not meet proper engineering standards.
Many existing platforms have flow lines, ladders or handrails attached to the platform legs which can prevent or impede the installation of a temporary support platform. These obstacles often must be removed which adds further costs to the project. The removal of flow lines can also lead to leaks causing environmental problems.
Some platforms and wells are old and proper drawings and engineering calculations may not be readily available. In such situations, a structural team must be sent to the well site to create as built drawings and to assess the level of corrosion and deterioration of the existing wellhead and platform and its ability to support a temporary support structure.
Guywires rigged to fixed platforms are often used in constructing these temporary support platforms. When a snubbing unit or other equipment is mounted directly on the casing string, there is a risk that the casing string will shift or fall due to the weight of the snubbing unit and associated equipment and hook loads. This shifting may cause the guywires to become slack causing the unit to lean to one side or topple over causing damage to equipment, injury to workers, and possible loss of well control.
There is a need for a device that will aid in the construction of a temporary support structure for workbaskets and other platforms for well workover operations that will help to resolve the problems currently associated with such temporary structures. There is also a need for a support structure that may be attached directly to the casing string with minimal tailoring and fabrication at the well site and that can be readily removed when work is completed. There is also a need for a support structure that will reduce the time and expense associated with platform or workbasket installation and minimize the risk of damage to the wellhead and the risk of injury to workers